# Lookout #
Lookout is a lightweight unit testing framework. Tests (expectations) can be
written as follows
expect 2 do
1 + 1
end
expect NoMethodError do
Object.invalid_method_call
end
Lookout is designed to encourage – force, even – unit testing best practices
such as
* Setting up only one expectation per test
* Not setting expectations on non-public APIs
* Test isolation
This is done by
* Only allowing one expectation to be set per test
* Providing no (additonal) way of accessing private state
* Providing no setup and teardown methods, nor a method of providing test
helpers
Other important points are
* A unified syntax for setting up both state-based and behavior-based
expectations
* A focus on code readability by providing no mechanism for describing an
expectation other than the code in the expectation itself
The way Lookout works has been heavily influenced by [expectations], by [Jay
Fields][]. The code base was once also heavily based on [expectations],
based at Subversion [revision r76][]. A lot has happened since then and all of the
work past that revision are due to [Nikolai Weibull][].
[expectations]: http://expectations.rubyforge.org
[Jay Fields]: http://blog.jayfields.com
[revision r76]: https://github.com/now/lookout/commit/537bedf3e5b3eb4b31c066b3266f42964ac35ebe
[Nikolai Weibull]: http://bitwi.se
## Installation ##
Install Lookout with
% gem install lookout
## Usage ##
Lookout allows you to set expectations on an object’s state or behavior. We’ll
begin by looking at state expectations and then take a look at expectations on
behavior.
### Expectations on State ###
An expectation can be made on the result of a computation:
expect 2 do
1 + 1
end
Most objects, in fact, have their state expectations checked by invoking `#==`
on the expected value with the result as its argument.
Checking that a result is within a given range is also simple:
expect 0.099..0.101 do
0.4 - 0.3
end
Here, the more general `#===` is being used on the `Range`.
`Strings` of course match against `Strings`:
expect 'ab' do
'abc'[0..1]
end
but we can also match a `String` against a `Regexp`:
expect %r{a substring} do
'a string with a substring'
end
(Please note the use of `%r{…}` to avoid warnings that will be generated when
Ruby parses `expect /…/`.)
Checking that the result includes a certain module is done by expecting the
`Module`.
expect Enumerable do
[]
end
This, due to the nature of Ruby, of course also works for classes (as they are
also modules):
expect String do
'a string'
end
This doesn’t hinder us from expecting the actual `Module` itself:
expect Enumerable do
Enumerable
end
As you may have figured out yourself, this is accomplished by first trying
`#==` and, if it returns `false`, then trying `#===` on the expected `Module`.
This is also true of `Ranges` and `Regexps`.
Truthfulness is expected with `true` and `false`:
expect true do
1
end
expect false do
nil
end
Results equaling `true` or `false` are slightly different:
expect TrueClass do
true
end
expect FalseClass do
false
end
The rationale for this is that you should only care if the result of a
computation evaluates to a value that Ruby considers to be either true or
false, not the exact literals `true` or `false`.
Expecting output on an IO object is also common:
expect output("abc\ndef\n") do |io|
io.puts 'abc', 'def'
end
This can be used to capture the output of a formatter that takes an output
object as a parameter.
You should always be expecting errors from – and in, but that’s a different
story – your code:
expect NoMethodError do
Object.no_method
end
Often, not only the type of the error, but its description, is important to
check:
expect StandardError.new('message') do
raise StandardError.new('message')
end
As with `Strings`, `Regexps` can be used to check the error description:
expect StandardError.new(/mess/) do
raise StandardError.new('message')
end
(Note that some of Ruby’s built-in error classes have slightly complicated
behavior and will not allow you to pass a `Regexp` as a parameter. `NameError`
is such a class. This may warrant further investigation into whether or not
this is a bug, but I’ll leave that up to the reader to decide.)
Lookout further provides a fluent way of setting up expectations on boolean
results. An object can “be”
expect Class.new{ attr_accessor :running; }.new.to.be.running do |process|
process.running = true
end
or “not be”
expect Class.new{ attr_accessor :running; }.new.not.to.be.running do |process|
process.running = false
end
or to “have”
expect Class.new{ attr_accessor :finished; }.new.to.have.finished do |process|
process.finished = true
end
or “not have”
expect Class.new{ attr_accessor :finished; }.new.not.to.have.finished do |process|
process.finished = false
end
On the same note
expect nil.to.be.nil?
and
expect Object.new.not.to.be.nil?
As not every boolean method “is” or “has” you can even
expect nil.to.respond_to? :nil?
The rules here are that all `Objects` respond to `#to`. After `#to` you may
call
* `#not`
* `#be`
* `#have`
* Any method whose name ends with `?`
A call to `#not` must be followed by a call to one of the three alternatives
that follow it in the list. `#Be` and `#Have` must be followed by a call to a
method.
### Expectations on Behavior ###
We expect our objects to be on their best behavior. Lookout allows you to make
sure that they are.
Mocks let use verify that a method is called in the way that we expect it to
be:
expect mock.to.receive.dial('2125551212').twice do |phone|
phone.dial('2125551212')
phone.dial('2125551212')
end
Here, `#mock` creates a mock object, an object that doesn’t respond to
anything unless you tell it to. We tell it to expect to receive a call to
`#dail` with `'2125551212'` as its formal argument, and we expect it to receive
it twice. The mock object is then passed in to the block so that the
expectations placed upon it can be fulfilled.
Sometimes we only want to make sure that a method is called in the way that we
expect it to be, but we don’t care if any other methods are called on the
object. A stub object, created with `#stub`, expects any method and returns a
stub object that, again, expects any method, and thus fits the bill.
expect stub.to.receive.dial('2125551212').twice do |phone|
phone.dial('2125551212')
phone.hangup
phone.dial('2125551212')
end
We can also use stubs without any expectations on them:
expect 3 do
s = stub(:a => 1, :b => 2)
s.a + s.b
end
and we can stub out a specific method on an object:
expect 'def' do
a = 'abc'
stub(a).to_str{ 'def' }
a.to_str
end
You don’t have to use a mock object to verify that a method is called:
expect Object.to.receive.deal do
Object.deal
end
As you have figured out by now, the expected method call is set up by calling
`#receive` after `#to`. `#Receive` is followed by a call to the method to
expect with any expected arguments. The body of the mocked method can be given
as the block to the method. Finally, an expected invocation count may follow
the method. Let’s look at this formal specification in more detail.
The expected method arguments may be given in a variety of ways. Let’s
introduce them by giving some examples:
expect mock.to.receive.a do |m|
…
end
Here, the method `#a` must be called with any number of arguments. It may be
called any number of times, but it must be called at least once.
If a method must receive exactly one argument, you can use `arg`:
expect mock.to.receive.a(arg) do |m|
…
end
If a method must receive a specific argument, you can use that argument:
expect mock.to.receive.a(1..2) do |m|
…
end
The same matching rules apply for arguments as they do for state expectations,
so the previous example expects a call to `#a` with 1, 2, or the Range 1..2 as
an argument on `m`.
If a method must be invoked without any arguments you can use
`without_arguments`:
expect mock.to.receive.a(without_arguments) do |m|
…
end
You can of course use both `arg` and actual arguments:
expect mock.to.receive.a(arg, 1, arg) do |m|
…
end
The body of the mock method may be given as the block. Here, calling `#a` on
`m` will give the result `1`:
expect mock.to.receive.a{ 1 } do |m|
…
end
If no body has been given, the result will be a stub object.
There is a caveat here in that a block can’t yield in Ruby 1.8. To work around
this deficiency you have to use the `#yield` method:
expect mock.to.receive.a.yield(1) do |m|
…
end
Any number of values to yield upon successive calls may be given. The last
value given will be used repeatedly when all preceding values have been
consumed. It’s also important to know that values are splatted when they are
yielded.
To simulate an `#each`-like method you can use `#each`. The following horrible
example should give you an idea of how to use it.
expect Object.new.to.receive.each.each(1, 2, 3) do |o|
class << o
include Enumerable
end
o.inject{ |i, a| i + a }
end
Invocation count expectations can also be set if the default expectation of
“at least once” isn’t good enough. The following expectations are possible
* `#at_most_once`
* `#once`
* `#at_least_once`
* `#twice`
And, for a given `N`,
* `#at_most(N)`
* `#exactly(N)`
* `#at_least(N)`
Method stubs are another useful thing to have in a unit testing framework.
Sometimes you need to override a method that does something a test shouldn’t
do, like access and alter bank accounts. We can override – stub out – a method
by using the `#stub` method. Let’s assume that we have an `Account` class that
has two methods, `#slips` and `#total`. `#Slips` retrieves the bank slips that
keep track of your deposits to the `Account` from a database. `#Total` sums
the `#slips`. In the following test we want to make sure that `#total` does
what it should do without accessing the database. We therefore stub out
`#slips` and make it return something that we can easily control.
expect 6 do |m|
account = Account.new
stub(account).slips{ [1, 2, 3] }
account.total
end
As with mock methods, if no body is given, the result will be a stub object.
To make it easy to create objects with a set of stubbed methods there’s also a
convenience method:
expect 3 do
s = stub(:a => 1, :b => 2)
s.a + s.b
end
Please note that this makes it impossible to stub a method on a Hash, but you
shouldn’t be doing that anyway. In fact, you should never mock or stub methods
on value objects.
## Integration ##
Lookout can be used from [Rake][]. Simply include the following code in your
`Rakefile`:
require 'lookout/rake/tasks'
Lookout::Rake::Tasks::Test.new
If the `:default` task hasn’t been defined it will be set to depend on the
`:test` task.
As an added bonus you can use Lookouts own [gem][RubyGems] tasks:
Lookout::Rake::Tasks::Gem.new
This provides tasks to `build`, `check`, `install`, and `push` your gem.
To use Lookout together with [Vim][], place `contrib/rakelookout.vim` in
`~/.vim/compiler` and add
compiler rakelookout
to `~/.vim/after/ftplugin/ruby.vim`. Executing `:make` from inside [Vim][]
will now run your tests and an errors and failures can be visited with
`:cnext`. Execute `:help quickfix` for additional information.
Another useful addition to your `~/.vim/after/ftplugin/ruby.vim` file may be
nnoremap M :call run_test()
let b:undo_ftplugin .= ' | nunmap M'
function! s:run_test()
let test = expand('%')
let line = 'LINE=' . line('.')
if test =~ '^lib/'
let test = substitute(test, '^lib/', 'test/', '')
let line = ""
endif
execute 'make' 'TEST=' . shellescape(test) line
endfunction
Now, pressing `M` will either run all tests for a given class, if the
implementation file is active, or run the test at or just before the cursor, if
the test file is active. This is useful if you’re currently receiving a lot of
errors and/or failures and want to focus on those associated with a specific
class or on a specific test.
[Rake]: http://rake.rubyforge.org
[RubyGems]: http://rubygems.org
## Interface Design ##
The default output of Lookout can Spartanly be described as Spartan. If no
errors or failures occur, no output is generated. This is unconventional, as
unit testing frameworks tend to dump a lot of information on the user,
concerning things such as progress, test count summaries, and flamboyantly
colored text telling you that your tests passed. None of this output is
needed. Your tests should run fast enough to not require progress reports.
The lack of output provides you with the same amount of information as
reporting success. Test count summaries are only useful if you’re worried that
your tests aren’t being run, but if you worry about that, then providing such
output doesn’t really help. Testing your tests requires something beyond
reporting some arbitrary count that you would have to verify by hand anyway.
When errors or failures do occur, however, the relevant information is output
in a format that can easily be parsed by an `'errorformat'` for [Vim][] or with
[Compilation Mode][] for [Emacs][]. Diffs are generated for Strings, Arrays,
Hashes, and I/O.
[Vim]: http://www.vim.org
[Compilation Mode]: http://www.emacswiki.org/emacs/CompilationMode
[Emacs]: http://www.gnu.org/software/emacs/
## External Design ##
Let’s now look at some of the points made in the introduction in greater
detail.
Lookout only allows you to set one expectation per test. If you’re testing
behavior with a mock, then only one method-invocation expectation can be set.
If you’re testing state, then only one result can be verified. It may seem
like this would cause unnecessary duplication between tests. While this is
certainly a possibility, when you actually begin to try to avoid such
duplication you find that you often do so by improving your interfaces. This
kind of restriction tends to encourage the use of value objects, which are easy
to test, and more focused objects, which require simpler tests, as they have
less behavior to test, per method. By keeping your interfaces focused you’re
also keeping your tests focused.
Keeping your tests focused improves, in itself, test isolation, but let’s look
at something that hinders it: setup and teardown methods. Most unit testing
frameworks encourage test fragmentation by providing setup and teardown
methods.
Setup methods create objects and, perhaps, just their behavior for a set of
tests. This means that you have to look in two places to figure out what’s
being done in a test. This may work fine for few methods with simple set-ups,
but makes things complicated when the number of tests increases and the set-up
is complex. Often, each test further adjusts the previously set-up object
before performing any verifications, further complicating the process of
figuring out what state an object has in a given test.
Teardown methods clean up after tests, perhaps by removing records from a
database or deleting files from the file-system.
The duplication that setup methods and teardown methods hope to remove is
better avoided by improving your interfaces. This can be done by providing
better set-up methods for your objects and using idioms such as [Resource
Acquisition Is Initialization][] for guaranteed clean-up, test or no test.
By not using setup and teardown methods we keep everything pertinent to a test
in the test itself, thus improving test isolation. (You also won’t [slow down
your tests][Broken Test::Unit] by keeping unnecessary state.)
Most unit test frameworks also allow you to create arbitrary test helper
methods. Lookout doesn’t. The same rationale as that that has been
crystallized in the preceding paragraphs applies. If you need helpers you’re
interface isn’t good enough. It really is as simple as that.
To clarify: there’s nothing inherently wrong with test helper methods, but they
should be general enough that they reside in their own library. The support
for mocks in Lookout is provided through a set of test helper methods that make
it easier to create mocks than it would have been without them.
[Lookout-rack][] is another example of a library providing test helper methods
(well, one of method, actually) that are very useful in testing web
applications that use [Rack][].
A final point at which some unit test frameworks try to fragment tests further
is documentation. These frameworks provide ways of describing the whats and
hows of what’s being tested, the rationale being that this will provide
documentation of both the test and the code being tested. Describing how a
stack data structure is meant to work is a common example. A stack is,
however, a rather simple data structure, so such a description provides little,
if any, additional information that can’t be extracted from the implementation
and its tests themselves. The implementation and its tests is, in fact, its
own best documentation. Taking the points made in the previous paragraphs into
account, we should already have simple, self-describing, interfaces that have
easily understood tests associated with them. Rationales for the use of a
given data structure or system-design design documentation is better suited in
separate documentation focused at describing exactly those issues.
[Resource Acquisition Is Initialization]: http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization
[Broken Test::Unit]: http://37signals.com/svn/posts/2742-the-road-to-faster-tests
[Lookout-rack]: http://github.com/now/lookout-rack
[Rack]: http://rack.rubyforge.org
## Internal Design ##
The internal design of Lookout has had a couple of goals.
* As few external dependencies as possible
* As few internal dependencies as possible
* Internal extensibility provides external extensibility
* As fast load times as possible
* As high a ratio of value objects to mutable objects as possible
* Each object must have a simple, obvious name
* Use mix-ins, not inheritance for shared behavior
* As few responsibilities per object as possible
* Optimizing for speed can only be done when you have all the facts
### External Dependencies ###
Lookout used to depend on Mocha for mocks and stubs. While benchmarking I
noticed that a method in Mocha was taking up more than 300 percent of the
runtime. It turned out that Mocha’s method for cleaning up back-traces
generated when a mock failed was doing something incredibly stupid:
backtrace.reject{ |l| Regexp.new(@lib).match(File.expand_path(l)) }
Here `@lib` is a `String` containing the path to the lib subdirectory in the
Mocha installation directory. I reported it, provided a patch five days later,
then waited. Nothing happened. 254 days later, according to [Wolfram
Alpha][254 days], half of my patch was, apparently – I say “apparently”, as I
received no notification – applied. By that time I had replaced the whole
mocking-and-stubbing subsystem and dropped the dependency.
Many Ruby developers claim that Ruby and its gems are too fast-moving for
normal package-managing systems to keep up. This is testament to the fact
that this isn’t the case and that the real problem is instead related to sloppy
practices.
Please note that I don’t want to single out the Mocha library nor its
developers. I only want to provide an example where relying on external
dependencies can be “considered harmful”.
[254 days]: http://www.wolframalpha.com/input/?i=days+between+march+17%2C+2010+and+november+26%2C+2010
### Internal Dependencies ###
Lookout has been designed so as to keep each subsystem independent of any
other. The diff subsystem is, for example, completely decoupled from any other
part of the system as a whole and could be moved into its own library at a time
where that would be of interest to anyone. What’s perhaps more interesting is
that the diff subsystem is itself very modular. The data passes through a set
of filters that depends on what kind of diff has been requested, each filter
yielding modified data as it receives it. If you want to read some rather
functional Ruby I can highly recommend looking at the code in the
`lib/lookout/diff` directory.
This lookout on the design of the library also makes it easy to extend Lookout.
Lookout-rack was, for example, written in about four hours and about 5 of those
240 minutes were spent on setting up the interface between the two.
### Optimizing For Speed ###
The following paragraph is perhaps a bit personal, but might be interesting
nonetheless.
I’ve always worried about speed. The original Expectations library used
`extend` a lot to add new behavior to objects. Expectations, for example, used
to hold the result of their execution (what we now term “evaluation”) by being
extended by a module representing success, failure, or error. For the longest
time I used this same method, worrying about the increased performance cost
that creating new objects for results would incur. I finally came to a point
where I felt that the code was so simple and clean that rewriting this part of
the code for a benchmark wouldn’t take more than perhaps ten minutes. Well,
ten minutes later I had my results and they confirmed that creating new objects
wasn’t harming performance. I was very pleased.
### Naming ###
I hate low lines (underscores). I try to avoid them in method names and I
always avoid them in file names. Since the current “best practice” in the Ruby
community is to put `BeginEndStorage` in a file called `begin_end_storage.rb`,
I only name constants using a single noun. This has had the added benefit that
classes seem to have acquired less behavior, as using a single noun doesn’t
allow you to tack on additional behavior without questioning if it’s really
appropriate to do so, given the rather limited range of interpretation for that
noun. It also seems to encourage the creation of value objects, as something
named `Range` feels a lot more like a value than `BeginEndStorage`. (To reach
object-oriented-programming Nirvana you must achieve complete value.)
## Contributors ##
Contributors to the original expectations codebase are mentioned there. We
hope no one on that list feels left out of this list. Please [let us
know][Lookout issues] if you do.
* Nikolai Weibull
[Lookout issues]: https://github.com/now/lookout/issues
## License ##
You may use, copy, and redistribute this library under the same [terms][] as
Ruby itself.
[terms]: http://www.ruby-lang.org/en/LICENSE.txt